1. Field of the Invention
The present invention relates to a developing device, an image developing method, an image forming apparatus, an image forming method and a process cartridge.
2. Description of the Related Art
Conventionally, in electrophotography, an image has been formed by developing a latent electrostatic image formed by charging or exposing a photoconductor surface with color toners to form a toner image, transferring the toner image onto an image transfer member such as a transfer sheet and fixing the toner image formed on the transfer sheet using a heat roller or the like.
Dry process developing methods employed in electrophotographic process and electrostatic recording etc. are divided into two streams, i.e., a method using a two-component developer containing a toner and a carrier and a method using a one-component developer containing no carrier. The former method, i.e., the method using a two-component developer allows for obtaining a favorable image in relatively stable condition, however, it is difficult to obtain images having constant quality over a long period of time because deterioration of carrier and a variation in mixture ratio of a toner and carrier and the like easily occur. Further, the method using a two-component developer is disadvantageous in controlling maintenance of developing devices and in making an developing device compact. For this reason, the latter method, i.e., the method using a one-component developer which does not have the disadvantage has been a focus of attention.
In the method using a one-component developer, a toner which serves as a developer is conveyed by a toner conveyance member and a unit configured to visualize a latent electrostatic image formed on a photoconductor using the toner is used. However, in the conveyance, a toner layer conveyed over the toner conveyance member surface must be sufficiently thinned. When a material having a high electric resistance is used as a toner, the toner needs to be charged using a developing device, and thus a toner layer must be particularly thinned. When a toner layer is thick, only the surface of the toner layer is charged and it is difficult for the entire toner layer to be charged evenly.
For this reason, for a unit used to adjust the thickness of a toner layer on a toner conveyance member (a toner layer thickness controlling unit), various methods have been proposed such as using a controlling member. As a primary example, there is a method in which a toner layer thickness controlling blade as a controlling member is placed so as to face a toner conveyance member and a toner conveyed over the surface of a toner conveyance member is pressed with the toner layer thickness controlling blade to thereby control the toner layer thickness. Further, a method is also proposed in which instead of the toner layer thickness controlling blade, a roller is made to contact with a toner to thereby obtain a similar effect.
In an image developing step, i.e., at the time of developing an image on a photoconductor, in a toner layer formed on the surface of a developing roller, which serves as a toner conveyance member, by the toner layer thickness controlling unit, a toner residing near the developing roller surface has an extremely high charge. Since the toner having an extremely high charge is strongly attracted to the developing roller surface, the transfer rate of the toner from the developing roller onto a latent electrostatic image on the photoconductor is reduced and a charge-up phenomenon easily occur.
When a charge-up phenomenon occurs, the surface layer of a toner layer formed on a developing roller surface becomes harder to be charged and the charged amount of the toner is reduced. As a result, background smear in non-image-formed portions, toner bleed, toner scattering and the like easily occur.
To prevent these phenomena, there is a need to control such that the charged amount of a toner layer formed on the surface of a developing roller is uniformed.
When a toner layer formed on a developing roller surface cannot be efficiently thinned and uniformly charging of the toner layer cannot be efficiently controlled, a phenomenon that a conveyance amount of a toner on the developing roller is increased more than necessary easily occurs in the course of continuation of printing to the developing device lifetime. Due to occurrence of the phenomenon, not only background smear in non-image formed portions, toner bleed and toner scattering occur but also nonuniformity of image formed portions is easily conspicuous.
Such a phenomenon tends to take place particularly when a power source of a copier is turned off overnight or longer and thereafter the copier is started up. The reason why the phenomenon tends to take place is that the adsorbability of a toner onto a developing roller is increased more than necessary because its high toner charged amount in restarting a copier, and then the toner conveyance amount is largely increased.
To solve these problems, it is necessary to prevent the toner charged amount from being excessively increased and to control the toner conveyance amount within a certain definite-range, if circumstances allow.
For example, Japanese Patent (JP-B) No. 2754539 proposed a method in which a tone conveyance amount is preferably controlled within a range of 0.5 mg/cm2 to 0.75 mg/cm2 so as to appropriately control the toner conveyance amount.
Similarly to the above method, for example, Japanese Patent Application Laid-Open (JP-A) No. 2004-279912 proposes a method in which a toner amount of a toner on a toner bearing member is controlled within a range of 0.2 mg/cm2 to 0.45 mg/cm2 and Japanese Patent Application Laid-Open (JP-A) No. 2004-279913 proposed a method in which a toner amount of a toner on a toner bearing member is controlled within a range of 0.45 mg/cm2 to 1.0 mg/cm2.
For a method of uniformly charging a toner on a developing roller, which is another approach to solve the above-noted problems, adding the following various treatment agents in a toner is known.
For the treatment agent, for example, Japanese Patent Application Laid-Open (JP-A) No. 2002-31913 proposes to use magnesium silicate minerals (attapulgite, sepiolite etc.).
For the treatment agent, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 3-294864 and 4-214568 respectively propose to use a silicone oil-treated magnesium silicate.
For the treatment agent, for example, Japanese Patent Application Laid-Open (JP-A) No. 11-95480 also proposes to use a toner coated with a silicate fine powder (magnesium silicate) at a coverage of 60% to 100%.
For the treatment agent, for example, Japanese Patent Application Laid-Open (JP-A) No. 11-184239 also proposes to use a titanium acid fine powder.
For the treatment agent, for example, Japanese Patent Application Laid-Open (JP-A) No. 2003-186240 also proposes to use a titania.
Further, as a lubricant to apply a contact portion between a developing blade and a developing roller, for example, Japanese Patent Application Laid-Open (JP-A) No. 2004-264428 proposes to use a spherically shaped polymer particle that has a particle diameter much smaller than the weight average particle diameter of the toner and has reverse polarity to that of the toner.
However, the method disclosed in JP-B No. 2754539 specifies combinations of external additives to be used, however, the method has a difficulty in controlling a toner conveyance amount within a specific range while preventing desorption and imbedding of various external additives.
The methods disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 2004-279912 and 2004-279913 respectively specify a mass ratio of a negatively chargeable silica fine particle and a positively chargeable silica fine particle. However, the methods respectively have a difficulty in controlling a toner conveyance amount within a desired range for the above-noted reasons. Even if a toner conveyance amount is controllable, the method disclosed in JP-A No. 2004-279912 has a difficulty in ensuring a sufficient developed amount of a toner with the use of the lower limit value of the specified range of mass ratio for the negatively chargeable silica fine particle and the positively chargeable silica fine particle and is likely to cause image density defects. The method disclosed in JP-A No. 2004-279913 has a difficulty in uniformly charging a toner with the use of the upper limit value of the specified range of mass ratio for the negatively chargeable silica fine particle and the positively chargeable silica fine particle and is likely to cause background smear in non-image formed portions, toner bleed and toner scattering. Further, it is difficult to prevent occurrence of image nonuniformity.
In the method disclosed in JP-A No. 2002-31913, a magnesium silicate mineral used as a treatment agent has a high moisture content, and the method is likely to cause charge defects even in normal use environments as well as to cause troubles attributable to charge defects such as background smear, toner bleed, toner scattering and the like.
The methods disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 3-294864 and 4-214568 respectively have problems of causing degradation in toner flowability and increases in charged amount by the use of the silicone oil and of causing toner conveyance defects and reductions in image density in a developing device.
In the method disclosed in JP-A No. 11-95480, when a negatively chargeable toner is used, there is a problem that a reversely charged toner is likely to be generated, which easily leads to occurrence of background smear. This is caused by physical property of magnesium silicate contained in the toner that magnesium silicate is likely to be positively charged by influence of magnesium oxide (MgO) which is likely to have a strongly positive charge, as instructed in “Relation of Electronegativity” (see Nippon Gazo Gakkai shi or “Journal of the Imaging Society of Japan” Vol. 39, No. 3 at page 259).
The method disclosed in JP-A No. 11-184239 has a problem that a leaked amount of charge is large because the electric resistance of the titanium acid fine powder is low. Therefore, background smear, toner bleed and toner scattering easily occur. Further, the titanium acid fine powder is prone to break away from a toner and when a contact-charge process is employed, the titanium acid fine powder causes contamination of the contact-charge members and charge defects of the used photoconductor, ending up with occurrence of image defects.
Further, the method disclosed in JP-A No. 2003-186240 has a difficulty in adjusting the additive amount of titania because titania is a material having electrically low resistance and a high dielectric constant. When a large amount of titania is added, the leaked amount of charge is large, which causes reduction in charged amount of the entire toner. In contrast, when the additive amount of titania is small, it causes an increase in charged amount. Thus, in either case, background smear, toner bleed and toner scattering are likely to occur. Further, when a titania having a relatively large particle diameter is used, the titania is prone to break away from the toner. Accordingly, when a contact-charge process is employed, titania causes contamination of the contact-charge members and charge defects of the used latent electrostatic image bearing member, ending up with occurrence of image defects.
Furthermore, the method disclosed in JP-A No. 2004-264428 uses a developing device using a one-component nonmagnetic toner, the developing device is not a developing device of which such a toner is applied to a controlling member (blade).